This invention is directed to motor control and more particularly to variable speed control of multiple air conditioning and refrigeration compressors.
Variable speed control of a motors, and more particularly, compressor motors in a refrigeration or air conditioning application is often accomplished using a variable speed drive. This allows the removal of all unloading hardware from the compressor system. In typical applications involving more than one compressor, such as multiple circuit chillers, multiplexed compressor chillers, refrigeration, a variable speed control could be used with each compressor to selectively unload compressors as necessary based on system demand. Variable speed drives are expensive and therefore, multiple compressor systems requiring multiple variable speed drives also become extensively more expensive. In addition, the need for multiple variable speed drives adds to the complexity and size of the air conditioning or refrigeration system.
There exists a need therefore, for a solution which allows for the reduction in the number of variable speed drives required for multi-motor or multi-compressor systems so as to reduce cost and system complexity.
An object of this invention is to provide a multiple motor, and particularly multiple compressor system, having a single variable speed drive for unloading the motors or compressor motors.
Another object of this invention is to provide a lower cost, less complex variable speed compressor systems with multiple compressors and motors, having a single variable speed drive.
Yet another object of the following invention is to provide a mechanism for maintaining the desired combined load during the unloading of one compressor motor to line power and loading of a second to variable speed, while accounting for the time to speed of the second motor.
In accordance with foregoing objects and following advantages, a system for variable speed control of a plurality of motors under variable load demand is provided. The system includes a constant power source and a plurality of motors having electrical circuits with switches to connect with the constant power source. The system further includes a variable speed drive for driving each of the plurality of motors at variable speed, wherein each of the plurality of motors includes a switch to connect with the variable speed drive. Each of the plurality of motors is selectively electrically connected with the variable speed drive and the constant power source via the switches, for driving the each of the plurality of motors at one of variable and constant speed in response to load demands. To prevent short cycling of the motors under situations of increasing system demand, the control algorithm for the electrical switches provides hysteresis by overspeeding one of the motors before switching from variable speed mode to constant speed mode. Similarly, hysteresis is included in the control algorithm to prevent short cycling of the motors under situations of decreasing system demand.